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C5.5–6 Salts from Insoluble Bases and Making More Salts

GCSE Combined Science · AQA · Chapter C5: Chemical Changes

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: making salts from metal oxides, hydroxides and carbonates, choosing reactants, and crystallising soluble salts. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Write word and balanced symbol equations for acids reacting with metal oxides, hydroxides and carbonates
  • Describe how to make crystals of a soluble salt from an acid and an insoluble base or carbonate
  • Choose an acid and a base to make a named salt, and work out the formula of a salt from the charges on its ions
  • Explain why an insoluble reactant is used to make a pure salt, and (Higher tier) calculate the mass of salt that can be made

Key words

  • Base: A substance that neutralises an acid to form a salt and water. Metal oxides and hydroxides are bases.
  • Alkali: A base that dissolves in water, forming OH−(aq) ions, e.g. sodium hydroxide.
  • Neutral- isation: The reaction between an acid and a base (or carbonate) that produces a salt.
  • Excess: More than is needed to react completely, so some is left over.
  • Filtrate: The clear solution that passes through filter paper.
  • Crystal- lisation: Evaporating some solvent and leaving a solution to cool so crystals form.

Acids with metal oxides and hydroxides

Metal oxides and metal hydroxides are bases. They neutralise acids to make a salt and water. Many metal oxides, such as copper(II) oxide, are insoluble, but they still react with acids, especially when warmed.

acid + metal oxide → salt + water CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l) (black solid → blue solution)

acid + metal hydroxide → salt + water NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l) Ca(OH)₂(aq) + 2HNO₃(aq) → Ca(NO₃)₂(aq) + 2H₂O(l)

Key fact: Every alkali is a base, but not every base is an alkali. An alkali is a base that dissolves in water.

Working out the formula of a salt

A salt is an ionic compound: positive metal ions (or ammonium ions, NH₄⁺) with negative ions from the acid. The salt has no overall charge, so the charges on its ions must add up to zero. Transition metals can form ions with different charges; the Roman numeral in the name gives the charge, e.g. iron(III) = Fe³⁺.

Worked example: formula of ammonium sulfate

Ions: NH₄⁺ (charge +1) and SO₄²− (charge −2).
To cancel −2 you need two +1 charges, so the ratio is 2 NH₄⁺ : 1 SO₄²−.
Put brackets round a group ion when you need more than one of it.

Answer: (NH₄)₂SO₄

Exam tip: Quick method: the number of each ion is the size of the other ion’s charge, then cancel if you can. Fe³⁺ and O²− → Fe₂O₃; Ca²⁺ and CO₃²− → Ca₂(CO₃)₂ → CaCO₃.

Acids with metal carbonates

Metal carbonates react with acids to make a salt, water and carbon dioxide. You see fizzing, and the solid disappears.

acid + metal carbonate → salt + water + carbon dioxide ZnCO₃(s) + 2HCl(aq) → ZnCl₂(aq) + H₂O(l) + CO₂(g) K₂CO₃(aq) + 2HNO₃(aq) → 2KNO₃(aq) + H₂O(l) + CO₂(g)

Most metal carbonates are insoluble in water; the main exceptions are the Group 1 carbonates (such as sodium carbonate) and ammonium carbonate. Farmers use this reaction when they spread powdered limestone (mainly calcium carbonate) on fields that are too acidic.

(Higher tier) For an insoluble carbonate the ionic equation shows the solid reacting with hydrogen ions, e.g. CuCO₃(s) + 2H⁺(aq) → Cu²⁺(aq) + H₂O(l) + CO₂(g). For a dissolved carbonate it is simply CO₃²−(aq) + 2H⁺(aq) → H₂O(l) + CO₂(g).

Key fact: Test for carbon dioxide: bubble the gas through limewater (calcium hydroxide solution). It turns milky (cloudy).

Making a soluble salt from an insoluble base

  • Add the solid in excess to warm dilute acid and stir. When some solid stays undissolved, all the acid has reacted, so no acid is left in the product. With a carbonate, fizzing stops when the acid is used up.
  • Filter to remove the unreacted solid. The filtrate is a solution of the salt in water.
  • Crystallise: heat the filtrate gently (a water bath stops spitting) until crystals start to form at the edge, then leave it to cool. Pat the crystals dry with filter paper.

You will plan and evaluate this method in detail in the required practical (C5 RP).

Why use an insoluble reactant?

Salts used in medicines, foods and fertilisers must be pure. It is easiest to make a pure salt from an acid and a reactant that is insoluble in water – a metal, metal oxide, metal hydroxide or metal carbonate. Because it does not dissolve, you can add it in excess to use up all the acid, then simply filter the leftover solid off. Nothing but the salt is left in the solution.

If the base is an alkali (soluble), any extra would dissolve and could not be filtered off, so the salt would be contaminated with alkali (or with acid, if too little were added). In every acid + alkali reaction the H⁺(aq) ions join with OH−(aq) ions: H⁺(aq) + OH−(aq) → H₂O(l). The other ions stay in solution as the salt.

  • copper(II) sulfate; Acid sulfuric acid; Insoluble reactant that could be used copper(II) oxide, hydroxide or carbonate (not copper metal – it is below hydrogen)
  • zinc nitrate; Acid nitric acid; Insoluble reactant that could be used zinc, zinc oxide or zinc carbonate
  • magnesium chloride; Acid hydrochloric acid; Insoluble reactant that could be used magnesium, magnesium oxide, hydroxide or carbonate
  • iron(II) sulfate; Acid sulfuric acid; Insoluble reactant that could be used iron (or iron(II) carbonate)

Exam tip: Hydrated crystals, such as blue copper(II) sulfate crystals (CuSO₄·5H₂O), contain water in their structure. If you evaporate a solution to dryness, the water of crystallisation is lost and you get a powder, not crystals.

How much salt? (Higher tier)

You can use the balanced equation and moles to work out the maximum mass of salt that a given mass of reactant can make.

Worked example: mass of salt from a metal oxide

CuO + H₂SO₄ → CuSO₄ + H₂O. 4.0 g of CuO reacts with excess acid. (Ar: O = 16, S = 32, Cu = 63.5)
Mr of CuO = 79.5, so moles of CuO = 4.0 ÷ 79.5 = 0.0503 mol
ratio 1 : 1, so 0.0503 mol of CuSO₄ can form
Mr of CuSO₄ = 63.5 + 32 + (4 × 16) = 159.5, so mass = 0.0503 × 159.5 = 8.03 g

Answer: 8.0 g of anhydrous copper(II) sulfate (2 s.f.)

In practice you always collect less than this: some solution is left on the filter paper and in the apparatus, some salt stays dissolved after crystallisation, and some crystals are lost when they are transferred.

Common mistakes

  • Forgetting carbon dioxide as a product when an acid reacts with a carbonate.
  • Saying the excess solid is added “to make more salt”. It is added so that all the acid reacts and none is left in the product.
  • Heating the solution until it is completely dry. This can spoil the crystals; heat until crystals start to form, then leave it to cool.
  • Calling every base an alkali. Copper(II) oxide is a base but it is insoluble, so it is not an alkali.

Revision questions

Try these, then check your answers below each one.

Question 1. What are the products when a metal carbonate reacts with a dilute acid? (Choose from: a salt, water and carbon dioxide, a salt and hydrogen, a salt and water only, a salt, hydrogen and carbon dioxide.)

Answer: a salt, water and carbon dioxide

Working: Acid + metal carbonate → salt + water + carbon dioxide.

Question 2. Name the salt made when zinc oxide reacts with dilute hydrochloric acid.

Answer: zinc chloride

Working: Zinc chloride: ZnO + 2HCl → ZnCl₂ + H₂O.

Question 3. When making a salt from an insoluble base, why is the base added in excess? (Choose from: to make sure all the acid reacts, to make more salt than the acid allows, to speed up crystallisation, to make the base dissolve in water.)

Answer: to make sure all the acid reacts

Working: Adding excess base uses up all the acid, so no acid is left in the salt. The extra solid is then filtered off.

Related lessons

Continue in the full course

This lesson is section C5.5–6 (AGCSC2205) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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